1. Field of the Invention
The present invention relates to an optical pick-up device for recording and reproducing information from an information record medium having plural record layers in direction of depth.
2. Related Art Statement
This kind of optical pick-up device has been described in for example Japanese Patent Application Laid-open No. 185640/1996. Such an optical pick-up device comprises an outward route optical system in which a laser light from a semiconductor laser is collimated to form a light flux by a collimator lens, afterwards, is reflected with a beam splitter and is irradiated on a multi-layer optical disc by an objective lens, and comprises an inward route optical system in which the reflected light after being transmitted through the beam splitter and the objective lens is received on an optical detector through a diaphragm lens and a pin-hole, thereby reproducing the information recorded on the desired record layer.
Herein, the diameter D of the pin-hole is made a value calculated by the following expression (1), assuming that in order to transmit the reflected light from the desired record layer and to irradiate it on the light detector incident, xcex7 is lateral magnification of the reflected light detection system, NAob is a numerical aperture of the objective lens, f1 is a focal distance of the objective lens, f2 is a focal distance of the diaphragm lens, d is a distance between respective record layers of multi-layer optical disk, and n is a refractive index of the disc substrate,
D=(⅕){xcex72xc2x7NAxc2x7f1)/(f2+2xcex72d/n)}(d/n)xe2x80x83xe2x80x83(1)
Therefore, assuming that for example, xcex7=10, NAob=0.55, f1=3 mm, f2=30 mm, d=0.007 mm, and n=1.56, the diameter D of the pin-hole becomes D≈4.8 xcexcm.
By the way, in the optical pick-up device as described above, assuming that r is a pin-hole radius, xcex is a utilizing wave length, NAdp is a numerical aperture of tie diaphragm lens, and Vp is a pin-hole diameter (radius of Airy image) standardized by the utilizing wave length xcex and the numerical aperture NAdp, Vp can be represented as described by the following expression (2) as shown in CONFOCAL MICROSCOPY (Wilson,T) 1990. ACADEMIC PRESS INC.
Vp=2xcfx80xc2x7r/(xcex/NAdp)xe2x80x83xe2x80x83(2)
Moreover, the profile of Airy image of the spot formed on pin-hole plane (intensity distribution) becomes as shown in FIG. 4 by thin line, and thus Airy disc radius becomes Vp=3.8, so that intensity integral value from the spot center becomes as shown in FIG. 4 by thick line.
Herein, assuming that for example xcex=0.68 xcexcn, numerical aperture NAdp of the diaphragm lens in the above case becomes NAdp=0.055, since the numerical aperture of the objective lens is NAob0.55, and f1/f2={fraction (1/10)}, so that the spot diameter of the luminous flux incident on the pin-hole plane becomes substantially 15 xcexcm from the above expression (2), at Vp=3.8.
Therefore, in the conventional optical pick-up device described above, assuming that pin-hole diameter D is 4.8 xcexcm (Vp at this time becomes Vp≈1.2) calculated by the above expression (1), the quantity of light transmitted through the pin-hole becomes below the half, and thus the utilizing efficiency of reflected light from the desired record layer of the multi-layer optical disc is decreased.
On the other hand, in the case of the recording medium having single layer, the reflected light from the recording medium is usually 10%-90%, and in the case of the recording medium having plural recording layers and recording the pit data on the recording layer by focusing the record beam to the position of desired depth in the medium which uses for example photo polymer, photo-refractive crystal, and photo-chromic material, etc. and by changing the refractive index and the light absorption coefficient thereof, the reflected light is very small and also 0.1% or less.
In such a way, in the conventional optical pick-up device described above, the light quantity incident on the photo detector becomes significantly decreased, so that the photo multiplexer and the avalanche photo diode each having current amplification function as photo detector must be used.
However, in case of using the photo detector having such a current amplification function, the high voltage is required, and the temperature compensation circuit is required, so that problems lie in that the circuit structure becomes complicated, and expensive and that the power consumption becomes increased, too.
The present invention has performed by considering the conventional problem particular.
The present invention has for its object to provide an optical pick-up device capable of efficiently receiving reflected light from the information record medium having the plural record layers in the direction of depth through the pin-hole, and capable of accurately reproducing the information recorded on the desired record layer.
According to the present invention, there is provided an optical pick-up device comprising: a semiconductor laser, an information record medium having plural record layers laminated thereon, an outward route system for irradiating laser light from the semiconductor laser on to the information record medium through a beam splitter and objective lens, and an inward route system having a light detector for receiving the light reflected from the information record medium transmitted through the objective lens, through the beam splitter and pin-holes; the pin-hole has a radius, which is decided in such a manner that an expression Vp=2xcfx80/xcex/NA) satisfies following condition 3xe2x89xa6Vpxe2x89xa76, wherein r is radius of the pin-hole, NA is lens aperture of the detection system, xcex is wave length of the laser light, and Vp is radius of Airy disc.
As has been seen from FIG. 4, intensity integral value in pin-hole plane becomes 80% or more in the range 3xe2x89xa6Vp, and becomes decreases rapidly in the range 3 greater than Vp. Therefore, if the radius of the pin-hole is set to satisfy the range 3xe2x89xa6Vp, the light amount incident on the optical detector can be increased, and the utilizing efficiency of reflected light from the information record medium can be improved.
However, Vp is made large, leakage-in of the signal from a record layer adjacent to the desired record layer becomes large, and thus the resolution may be decreased. FIG. 5 shows intensity distribution in the direction of focal depth of the objective lens capable of being limited for the pin-hole radius Vp shown in FIG. 4. Herein, parameter u plotted on abscissa is a parameter shown by normalizing the value in the direction of focal depth with the utilizing wave length xcex and the numerical aperture NAob (NAob=sin xcex1) of the objective lens, with the use of the following expression (3). Moreover, z shows an actual focal depth. Moreover, this depth is shown by an air converted length, for the sake of simplicity. That is, actually, the plural record layers are formed by intervening a transparent plastic substrate, so that The refractive index of the plastic substrate should be considered for a focal depth, but for convenience of explanation, the focal depth is designated by a length in the state without plastic substrate, that is, a length converted into the refractive index (1.0) of the air.
U=(8xcfx80/xcex) z sin2(xcex1/2)xe2x80x83xe2x80x83(3)
In the expression (3), assuming that xcex=0.68 xcexcm, the parameter u becomes as shown in following table.
Here, if the spacing between record layers of the information record medium is 5 xcexcm, it is preferable to make focal depth z to be xc2x12.5 xcexcm or less, and the parameter becomes u=8 or less in the case of NAob=0.55. Moreover, in this case, it is understood that the leakage-in amount of the signal from an adjacent record layer becomes as shown in FIG. 6 if this reads from the intensity distribution in parameter u=8 of FIG. 5, and the leakage-in amount increases rapidly with the condition Vp greater than 6.
From the above, as described in claim 1, by setting pin-hole radius r to satisfy conditions 3xe2x89xa6Vpxe2x89xa66, the utilizing the efficiency of reflected light from the information record medium is improved, the quantity of light incident on the photo detector can be ensured enough and the resolution in the layer direction of the information record medium is improved, so that the reading of information recorded in the desired record layer becomes possible with high resolution.
According to the invention of claim 4, in the optical pick-up device described in claim 1, the collimator lens is arranged between the semiconductor laser and the objective lens.
According to the invention of claim 2, the laser light from the semiconductor laser is collimated to form the parallel light by a collimator lens, thereby becoming infinite optical system incident on the objective lens, so that the laser light can be effectively converged and by arranging the beam splitter in the optical path of the parallel light between for example the collimator lens and the objective lens, the influence by the angle dependency of incident light on the beam splitter can not suffered.
According to the invention of claim 6, in the optical pick-up device described in claim 1, the optical detector consists of a PIN photo diode.
According to the invention of claim 3, the circuit structure can be made easy and cheap by using the PIN photo diode, and the power consumption can be decreased.
According to other embodiment of the present invention, the optical pick-up device comprises a detection optical system and a servo optical system, said detection optical system comprising a first semiconductor laser, an information record medium having plural record layers laminated thereon, a first outward route system for irradiating laser light irradiated from the first semiconductor laser on to the information record medium through wavelength selection element and objective lens, and a first inward route system having a first light detector for receiving the light reflected from the information record medium by transmitting through the objective lens, the wavelength selection element and the pin-hole; and said servo optical system comprising a second semiconductor laser, an information record medium having plural record layers laminated thereon, a second outward route system for irradiating the laser light irradiated from the second semiconductor laser onto the information record medium through the beam splitter, the collimator lens, the wavelength selection element, and the objective lens, and a second inward route system having a second light detector for receiving light reflected from the information record medium by transmitting through the objective lens, the wavelength selection element and the beam splitter.